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The Rieske proteins of two phylogenetically distant acidophilic organisms, i.e. the proteobacteriumThiobacillus ferrooxidans and the crenarchaeonSulfolobus acidocaldarius, were studied by EPR. Redox titrations at a range of pH values showed that the Rieske centers of both organisms are characterized by redox midpoint potential-versus-pH curves featuring a common pK value of 6.2. This pK value is significantly more acidic (by almost 2 pH units) than that of Rieske proteins in neutrophilic species. The orientations of the Rieske center's g tensors with respect to the plane of the membrane were studied between pH 4 and 8 using partially ordered samples. At pH 4, theSulfolobus Rieske cluster was found in the "typical" orientation of chemically reduced Rieske centers, whereas this orientation changed significantly on going toward high pH values. TheThiobacillus protein, by contrast, appeared to be in the "standard …